/Engine/Source/Runtime/Engine/Classes/Particles/ParticleSystemComponent.h
Get the beam source tangent
true EmitterIndex and SourceIndex are valid - OutTangentPoint is valid false EmitterIndex or SourceIndex is invalid - OutTangentPoint is invalid
virtual bool GetBeamSourceTangent ( int32 EmitterIndex, int32 SourceIndex, FVector & OutTangentPoint ) const